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Police say an explosion at a chemical factory in Japan has killed at least five people and injured a dozen more.

The blast happened in the early afternoon at the plant in Yokkaichi, central Japan, when maintenance crews were working on a heat exchanger used in the production of silicon products, a plant spokesman said.

Initial reports put the death toll at two, but they were quickly revised upwards.

"Five people are dead. Twelve people have been injured, of whom nine sustained only minor injuries," said a spokeswoman at Mie prefectural police.

A separate police spokesman said the plant, run by Mitsubishi Materials, makes parts for solar panels and automobiles, dealing with polymers made from silicone, hydrogen and chlorine.

"The explosion occurred but there is no fire. We received an emergency call at 2:09 pm (local time)...and at 2:21 pm the incident appeared to have calmed down," he said.

Hiroki Morofuji, an official at the plant, said the incident had involved maintenance workers at the plant, sited in a heavily industrialised region.

Mitusbishi Materials, headquartered in the Japanese capital, makes a range of products including auto parts, silicon wafers for memory chips used in consumer electronics, and cement for road and bridge construction.

The company, which reported sales of about $12.2 billion in its latest fiscal year, has over 22,000 employees and operations across the globe including in the United States, Brazil, Germany and India, according to its website.